In electric motor applications it is frequently desirable to have precise control over the dynamic operation of the motor. The specific parameter being controlled may be the motor speed, the motor shaft position, or the motor torque output. Until recently, DC motors have been almost exclusively used in these applications because their flux and torque can be easily controlled by controlling the field and armature currents of such a motor. DC motors, however, have limitations imposed by their commutators and brushes, including the need for periodic maintenance, and restrictions on the range of operation caused by the limited capability of the commutator and brushes to conduct current and to withstand high-voltage operation.
Induction or AC motors have no brushes and commutators and therefore do not suffer from these limitations, but until recently, induction motor controller circuits have been complicated, expensive and less accurate than DC motor controllers. Although induction motors were invented by Tesla in the 1890's, the dynamics of the operation of induction motors have not been truly understood until relatively recently. With the development of relatively inexpensive computers and digital control systems, the potential for increased productivity and lower cost has created a large demand for induction motor control circuits which can accurately control motor operation.
To provide accurate control of a motor's speed or position, the motor controller circuit must provide rapid and accurate control of the motor torque output in response to a torque command input signal. Recent research has discovered that an induction motor may be modelled as shown in FIG. 1A. In FIG. 1A, two input parameters, I.sub.F and I.sub.T control the rotor flux, motor torque, and motor slip frequency, w.sub.s. L.sub.m, w.sub.r, and R.sub.r respectively represent the magnetizing inductance, rotor natural frequency, and rotor resistance of the induction motor. If the inverse of the motor model shown in FIG. 1A is created, the diagram of FIG. 1B results. FIG. 1B represents a motor controller circuit having two inputs which determine the rotor flux and the motor torque produced by the motor. The outputs from the circuit of FIG. 1B are I.sub.F and I.sub.T, which may be used to control the stator current applied to the motor, as described in more detail below, and a value representing the slip frequency of the motor. A detailed explanation of the derivation of the diagrams shown in FIGS. 1A and 1B may be found in "An Approach to Flux Control of Induction Motors Operated With Variable Frequency Power Supply", by A. Nabae, et. al., IAS/IEE Annual Record 1978, pgs. 890-896.
To construct a simple and practical motor controller, it is desirable to be able to produce an output torque from the motor in response to and as a linear function of an input torque command signal. It is a basic principal of networks that given a transfer function relating output values to input variables, the inverse of that function will produce the input variables required to produce a desired output. This is shown diagrammatically in FIG. 1C. If a system has a transfer function F, relating the output states A and B of the system to input variables X and Y, the inverse, F.sup.-1, of the function will predict the inputs X and Y required to provide output states A and B. In response to inputs representing a desired torque and rotor flux, the transfer function represented in FIG. 1B will provide the quadrature components I.sub.F and I.sub.T necessary to produce the desired torque at the commanded rotor flux and the slip frequency w.sub.s which is also generated by the transfer function shown in FIG. 1B.
A few motor controllers have been developed based on the model of an induction motor shown in FIG. 1B. These controllers are sometimes known as flux-feed-forward controllers and depend upon using a network which is an accurate inverse of the motor being controlled. See, for example, U.S. Pat. Nos. 4,259,628 and 4,259,629. Presently available induction motor controllers, however, have limitations, particularly with respect to the bandwidth over which the motor dynamic response may be controlled, especially in applications where an induction motor is controlled by a digital control system having a limited data throughput capacity.